1. Field of the Invention
This invention relates to nuclear reactor containment arrangements, and more particularly to permanent seal rings extending across an annular thermal expansion gap between a peripheral wall of a nuclear reactor vessel and a containment wall wherein the seal ring provides a water tight seal across the expansion gap allowing for lateral and vertical translation of the reactor vessel relative to the containment wall.
2. Description of the Prior Art
Refueling of pressurized water reactors is an established routine operation carried out with a high degree of reliability. For normal load requirements, refueling is provided approximately every 12 to 22 months. The complete refueling operation normally takes approximately four weeks.
In a number of nuclear containment arrangements the reactor vessel is positioned within a concrete cavity having an upper annular portion above the vessel which defines a refueling canal. The canal is maintained dry during reactor operations; however, during refueling of the nuclear power plant, the canal is filled with water. The water level is high enough to provide adequate shielding in order to maintain the radiation levels within acceptable limits when the fuel assemblies are removed completely from the vessel. Boric acid is added to the water to ensure sub-critical conditions during refueling. At the beginning of the refueling operation, before the refueling canal is flooded, the reactor vessel flange is sealed to the lower portion of the refueling canal. Originally, this seal was achieved by a clamped gasket seal ring which prevents leakage of refueling water to the well in which the reactor vessel is seated. This gasket was fastened and sealed after reactor cool down prior to flooding of the canal. Typically this removable seal ring was made up of four large diameter elastomer gaskets which are susceptible to leakage and must be replaced at each refueling operation.
The annular thermal expansion gap between the reactor vessel and the concrete wall surrounding the reactor vessel is provided to accommodate thermal expansion of the vessel and other vessel movements such as in a seismic event and permit cooling of the cavity walls and the excore detectors embedded within the concrete cavity walls. Pressurized water reactor plants have two basic expansion gap sizes, i.e., wide and narrow. The wide gaps tend to be in the range of two to three feet, while narrow gaps tend to be in the range of two to four inches. In all plants, the gap area must be sealed during refueling. While the upper portion of the cavity, i.e., the refueling canal may be flooded, no water is permitted in the lower portion of the cavity. Typically, the reactor vessel has a horizontally extending flange and the containment wall surrounding the reactor cavity has a horizontally extending ledge or shelf at the floor of the refueling canal at about the same elevation as the flange, which the temporary seal ring spanned during refueling.
In plants with wide thermal expansion gaps permanent seal rings such as those described in U.S. Pat. Nos. 5,323,427; 4,905,260; 4,904,442; 4,747,993; and 4,170,517 have been employed to reduce the time required for the refueling operation. However, permanent seal rings need to allow for thermal expansion of the reactor vessel that reduces the gap between the peripheral wall of the reactor vessel and the containment wall in the area of the reactor well, and also ideally accommodate some vertical and lateral movement of the reactor vessel relative to the containment wall. In addition, the seal ring should be able to withstand heavy blows from objects such as fuel assemblies, accidentally dropped during refueling. A permanent seal ring also must permit the passage of cooling air from the lower reactor cavity along the wall surrounding the vessel up to the refueling canal. Thus, the seal ring must have (1) strength to retain the large volume of water used in the refueling operation; (2) flexibility to accommodate movement of the reactor vessel within the containment wall; (3) structural integrity to resist damage from falling objects; and (4) an air path between the lower reactor cavity and the refueling canal for cooling of the cavity walls during reactor operation. The foregoing patents describe various designs that achieved these objectives for wide gap plants; however, do not satisfy all of these objectives for narrow expansion gap plants. There are approximately 40 plants in the United States that have narrow expansion gaps that could take advantage of a permanent seal fixture if a design was available that could achieve all of the foregoing objectives.
Accordingly, it is an object of this invention to provide such a design that will satisfy all of the foregoing objectives for narrow expansion gap plants.